Solar energy is a promising alternative to hydrocarbon based energy production. There are various types of solar energy collection systems that are known in the art, which generally fall exclusively into one of two categories: photovoltaic technology and solar thermal technology. Generally, photovoltaic technology utilizes photovoltaic cell array to convert solar energy into electricity. Alternatively, solar thermal technology utilizes a working fluid to absorb solar energy then transfer the absorbed energy to electrical energy using a heat engine.
Photovoltaic cells utilizes the photovoltaic effect in which incoming solar energy, in the form of photons, are absorbed by the electrons of a semiconducting crystalline structure. If the energy level of the incoming photon, which corresponds to the wavelength of the incoming photon, surpasses that of the excitation level of the electron (i.e., the bandgap potential) the electron will absorb the photon and excite to a higher level of energy. The excited electron is then a vehicle for electrical energy transfer. The output of photovoltaic cells is direct current electricity which can directly power electrical equipment or stored in a battery.
Solar thermal technology uses the solar energy to heat a working fluid (e.g., salt water or molten salt) which is then used for either heating purposes elsewhere, or if temperatures are high enough, in a heat engine and generator producing electricity. In order for temperatures to reach useful heat engine temperatures, such as that needed to power a steam turbine, a solar concentrating structure is typically needed to intensify the incoming solar energy. Commonly used solar concentrating structures are reflective troughs and dishes as well as various lenses.
A common issue in both types of solar power collection is efficiency. In photovoltaic technology, one of the greatest efficiency losses is due that inability to utilize all of the energy from photons having various wavelengths of the solar spectrum. As previously mentioned, a particular semiconducting crystalline structure of a photovoltaic cell absorbs some portion of photons incident thereon having a certain wavelength or shorter, yet the amount of energy absorbed from each photon-electron interaction is limited to the energy of the bandgap potential. Thus, the excess energy present in a photon having a wavelength that is shorter than that of the bandgap potential of the electron is lost. Further, all photons having wavelengths that are longer, i.e. lower energy, than that needed to excite the electron are lost in their entirety, and typically pass through the semiconductor. To combat this loss of efficiency, conventional improvements in the art have included stacking semiconducting crystalline structures having different bandgap potentials in attempt to better capture a greater spectrum of solar light. Cost and technical feasibility, however, limits the number of semiconducting crystalline layers still leaving a large portion of the solar spectrum unused.
Solar thermal, on the other hand utilizes the entire solar spectrum, at least to the extent that each photon can transfer energy in the form of heat to a working fluid. However, the efficiency of transferring short wavelength light into electricity is lower than the efficiency of a photovoltaic cell designed to absorb that particular wavelength of light. Thus, solar thermal falls short of the energy conversion efficiencies of photovoltaic cells with respect to certain wavelengths.
To remedy these inefficiencies, hybrid solar collection devices have been developed. For instance, U.S. Pat. No. 8,455,755 to Correia, which is incorporated herein by reference, utilizes a reflective trough that focuses sunlight to a common point where a photovoltaic strip absorbs a portion of the incident solar spectrum for electrical conversion and absorbs the rest as thermal energy which is conducted to a thermal energy collector. While Correia utilizes both photovoltaic and solar thermal technologies to capture a greater efficiency rate, the thermal energy captured by the device must be absorbed and transferred through the photovoltaic strip inferring that the photovoltaic strip must operate at fairly high temperatures which negatively affects their efficiencies.
Thus, there are inefficiencies present in both photovoltaic and solar thermal technologies and there is a need in the industry to improve these inefficiencies in order for solar energy to remain a viable alternative to hydrocarbon energy.